The good news is that layout looks very easy to replicate. I don't think having both menus on screen at the same time will be possible though, but Pieter will have to confirm that. Even if that were the case having a horizontal main menu and a vertical text menu for the game list would be close and simple enough for him to navigate.
I'd offer to help but I have too many WIP projects as it is. I am a bit curious as to why you've decided to not just use RetroArch itself for his setup. I assume there must be a few RetroFE features you want to utilize which RA is lacking, otherwise it would be less work to just use RA directly.